I don't know what it means, but I've noticed that in the past few years much less is being made of some of the old American festivals. I was thinking of Labour Day here, the first Monday in September, which marks – on the calendar anyway, if not in the weather – the end of summer and the return to work.

Labour Day started by some New York Irish up in the late 1880s to celebrate what a century ago they called the labouring poor. It caught on throughout the rest of the country and for all the early and most of the middle years of this century, Labour Day was a big ceremonial affair in all the big cities, always a long parade of all the union locals winding up in the city central square and the big speech from the most important labour leader you could get, bands, drum majorettes strutting and twirling and in many cities – especially famous cities on the rivers – fireworks at night and the general happy feeling that right had been done by the American working classes.

Well, two things have happened. One is that fewer and fewer Americans, whatever their working lives, are willing to define themselves as working class. The last survey I saw, over 85% of Americans called themselves middle-class – an obvious impossibility in any society that has two recognisably substantial ends and a middle. The other thing more blatantly obvious down the past 20 years or so is the decline of the national unions, a turn of history I for one would have thought impossible if anyone had predicted it before or during the great organising campaigns of the Second World War. Today, only one American in five belongs to a union.

But Thanksgiving, the first, the cardinal American festival commemorating the first successful harvest of the original Massachusetts colony, Thanksgiving retains all its pristine colour and hold on the people. It's the great family reunion, more so for more Americans than the Christian Christmas. The original celebratory foods, the novelties the Indians introduced to the colonists are served from one end of Alaska to the Florida Keys – the turkeys, the corn pudding, the cranberry sauce, the pumpkin pie. Thanksgiving came and went this year in the shock of an anniversary so unexpected, so unproclaimed, so bizarre that it's only now a week or two later that I realise it was a unique anniversary and one well worth talking about.

A 50th anniversary not many people are alive to celebrate. The 50th anniversary of anything in their personal memory including their own wedding and here in New York, the immigrant haven for most of the past century too many of the 50th anniversary recollections were grievous ones of family and friends slaughtered in the Holocaust. But this 50th anniversary was of a film, a motion picture and showings of it exploded on Thanksgiving night across the country like the 4 July firework displays. This was not an act of studio hype, not a promotion racket for the company that made it. Warner Bros were dead long ago, but during the past past or four years film scholars busy about their separate specialities have found themselves bumping into each other and coming on surveys and memoirs about this one film.

And this year, it was discovered that this film remains the most popular film to most Americans over the past 50 years that three generations have seen it more often than any other film and mean to see it over and over again that no other film in history has registered more of its dialogue on the minds of movie goers and that all questions of merit aside, the film represents more than any other an American character or a duality in the American character that most Americans would like to believe is their true self, tough on the outside, compassionate within.

The film was shown first in New York on Thanksgiving night 1942 and the timing of its release as we shall see gave the Warner brothers cause to bow down and be doubly thankful to the lord. And the picture, Casablanca. It was filmed in just over two months through a hideous southern California heatwave in the summer of 1942 when air conditioning was not an amenity of American life. When it was all done with, nobody connected with it was very sorry, there was no studio party. The very opening scene of the movie an anxious scramble of European refugees and a gaggle of accents was one of the last things to be shot. The last day of all was what they call a cleaning up day, odd close-ups of the two of the films three stars Ingrid Bergman and Paul Henreid in a street scene that would be spliced into an early scene in the movie. Bogart was gone for good off on his boat, he had struck no sort of chord with Miss Bergman nor she with him, but they were pros and they played their parts.

As for Paul Henreid, the noble resistance leader, he had a habit of bad mouthing his co-stars, he thought Bogart a mediocre actor and implied as much with generally haughty behaviour. Poor Ingrid Bergman more than anybody the victim of a script that had eight different endings written for it never knew until the last days of shooting, which ending was for keeps and therefore, which man she was meant to love. The director, the veteran Hungarian Michael Curtiz who in 16 years in America had never got on easy speaking terms with English, howled impossible oaths at the bit players and fawned indiscriminately all over the stars in four languages. At the end, some thought they had a good film, some an indifferent one, but good or bad, special or routine; nobody had the sense of having produced an immortal. It started the day after the Japanese bombed Peal Harbor, a very big point for the making of this movie, If it had been made while the United States was still officially neutral, the sting of the plot would have been plucked, the Nazis would have been less vicious, Bogart might have remained a man who sticks his neck out for nobody, for the American studios were still selling their films in Germany right through 1941.

But on that Monday morning 8 December 1941, there had dropped on to the desk of a script reader at Warner Bros the manuscript of a play never performed and up for grabs by any studio that would buy it. It was the work of a young school teacher who'd spent the summer of 1940 in Austria, two memories stayed with him the behaviour of the Nazis in occupied Vienna and the smoky nightclub run by an American exile who had a black man for his piano player. Well the script was read by a hack reader who earned $1.12 cents an hour and he reported it to have the makings of an excellent melodrama, tense, tense mood, timely background. Casablanca in French Morocco where European refugees fought and bargained for exit visas to Portugal and America, sophisticated hokum a natural for Bogart or Cagney or Raft and perhaps Mary Astor.

Well, it was bought outright, to the school teacher's subsequent pain for $20,000 by Hal Wallis the studio's shrewdest producer. After weeks of shooting, Mr Wallis sent a memo instructing everybody that the movies title would be changed, from now on it would not be Everybody Comes to Rick's, but for no reason at all Casablanca. A very simple story about an expatriate American with a dubious past who runs a nightclub in Casablanca under the tolerant eye of a cynical French soldier, a captain, this is Vichy France, its a hotbed of intrigue, anxiety and barely repressed panic among refugees of all sorts of Hitler's Europe who are all desperately seeking exit visas to America.

A famous Resistance hero appears and shortly thereafter an SS major in the hunt for him, the American could help him. The major is determined to trap him. And the question, which sets the tension of the whole movie, is whether or not beneath the scar tissue of the Americans cynicism there is a whole man and an honourable one. Like all first-rate films apart from aesthetic considerations, Casablanca is a spring of slowly coiling intention, it is about evil and the risks in a bad time and place of resisting or overcoming it, and who shall overcome.

Well the film appeared when Hitler had throughout a decade demonstrated in Europe the success of violence, what sort of hero could hope to triumph over a Hitler acting out plots more monstrous than any the Warner brothers had contrived for their gangsters? No longer Leslie Howard or Ronald Coleman, not any of the suave white knights nor the simply naive giants of the prairie, the Gary Coopers and John Waynes. What was needed and what suddenly appeared as perfect for the time was a tough guy a cynic a subtle as Goebbels, but on our side. It would have shocked Bogart probably into stardom at almost anytime since the start of the Second War. The movie was meant to come out in the spring of 1943, but on 8 November 1942, we woke up to banner headlines "Americans invade French North Africa at Casablanca". The Warner brothers knelt and gave thanks.

The film was rushed into New York for Thanksgiving night coinciding with the first big military movement of the Americans in the European African Theatre, the film practically signified that America was now officially at war with Hitler on the home ground. There were 75 bit players in Casablanca and when the great scene was over in which the refugees in the cafe drown out the Nazis singing The Watch on the Rhine the cameraman noticed that half of the cast was in tears, they were all actual refugees, mostly Jewish with dreadful family records of slaughter in the concentration camps, they were not Americans from central casting faking accents.

I don't think its fanciful to think that the reality of that joyous moment passed over to the audiences and was responsible for the most extraordinary scene I was part of in a vast Philadelphia theatre, when Major Strassa was defeated and the Marseillaise flooded the soundtrack, the whole audience rose and cheered through its tears.

THIS TRANSCRIPT WAS TYPED FROM A RECORDING OF THE ORIGINAL BBC BROADCAST (© BBC) AND NOT COPIED FROM AN ORIGINAL SCRIPT. BECAUSE OF THE RISK OF MISHEARING, THE BBC CANNOT VOUCH FOR ITS COMPLETE ACCURACY.

Letter from America audio recordings of broadcasts ©BBC. Letter from America scripts © Cooke Americas, RLLP. All rights reserved.